1. Field of the Invention
The present invention relates to a water-cooled type cooling device that cools an engine block by circulating cooling water between the block and a radiator. More particularly, the present invention pertains to an apparatus for controlling an electrically operated cooling fan, which forcibly cools a radiator, in accordance with the temperature of the cooling water.
2. Description of the Related Art
An automobile engine is typically provided with a water-cooled type cooling apparatus. As shown in FIG. 9, such an apparatus includes a radiator 41, which transfers heat, a pump 42, which sends out pressurized cooling water, a thermostat 43, and pipes 44. When the engine is running, the pump 42 is activated to circulate cooling water through an engine block 45, the radiator 41, the thermostat 43, and the pipes 44. The circulation of the cooling water causes the heat of the block 45 to be transferred to the cooling water and cools the block 45. The heat of the cooling water is released into the ambient air by the radiator 41.
A typical radiator 41 is mounted at the front of an automobile 46. This enables an air stream, produced when the automobile 46 is moving, to cool the radiator 41. This, in turn, cools the cooling water passing through the radiator 41. A cooling fan 47 is provided adjacent to the radiator 41 to forcibly send a cooling current, which is required for heat transfer, to the radiator 41. When the car is stopped or when the air stream is insufficient, the fan 47 is rotated to cool the radiator 41.
A direct-drive type fan, which is driven by an engine's crankshaft, or an electrically operated fan, which is driven by an electric motor, is typically employed as the cooling fan. When the direct-drive type fan is used, the fan's rotating speed depends on the engine speed. Therefore, the flow rate of the air current produced by the fan does not necessarily correspond to the running condition of the engine. Contrarily, when an electrically driven fan is used, the fan's rotating speed is not dependent on the engine speed. Hence, it is possible to have the electrically operated fan produce an air current, the flow rate of which corresponds to the running condition of the engine. In addition, since the fan may be stopped when cooling is not required, the electrically operated fan is advantageous in that fan noise is not produced when the fan is stopped. Furthermore, since the electrically operated fan is separate from the engine, its location is not restricted by the location of the crankshaft.
An apparatus for controlling such an electrically operated fan is described in Japanese Unexamined Patent Publication No. 58-96119. This apparatus is shown in FIG. 10. The apparatus has a computer 51. The computer 51 controls the electric power supplied to a motor 53 of an electrically operated fan 52 from a battery 54. The detected values of the cooling water temperature and the running condition of the engine are input into the computer 51. The cooling water temperature is detected by a cooling water temperature sensor provided near the cooling water outlet of a radiator (not shown). When the cooling water temperature becomes equal to or higher than a predetermined upper limit value the computer 51 actuates a drive circuit which includes transistors TR1, TR2, TR3 and energizes the motor 53. When the cooling water temperature becomes lower than a predetermined lower limit value, the computer 51 de-energizes the motor 53. The computer 51 alters the value of the upper limit within a predetermined range in accordance with the running condition of the engine. Such structure enables the fan 52 to be rotated in accordance with various running conditions of the engine and allows optimal adjustment of the cooling water temperature.
The apparatus of the above publication may be employed in the cooling apparatus of FIG. 9. In such a case, the thermostat 43 is opened slightly when the cooling water temperature in the radiator 41 is lower than a predetermined value. This maintains the cooling water temperature measured near the cooling water outlet of the radiator 41 at a substantially constant value or at a temperature that changes slightly. In this state, the computer 51 operates the fan 52 if the cooling water temperature exceeds the predetermined upper limit value. Therefore, the computer 51 does not stop rotation of the fan 52 unless the cooling water temperature falls below the lower limit value regardless of whether the forced cooling causes the cooling water temperature to fall to a value close to the lower limit. Thus, the supply of electric power from the battery 54 to the motor 53 continues and the fan 52 keeps rotating. This causes unnecessary operation of the motor 53 and increases the power consumption of the motor 53. As a result, the electrical load on the alternator is increased. This increases the load on the engine and may decrease the engine's fuel consumption. In addition, unnecessary fan rotation prolongs the fan noise.